Ubuntu wiki states that its packages are compiled with secure flags and secure kernel options. I cannot find this information about debian. See https://wiki.ubuntu.com/CompilerFlags
I don't think Debian has anything exactly comparable to this. Here is Chapter 10 of Debian Policy: Files, which has some information about compile time flags. Note the sentence
It is up to the package maintainer to decide what compilation options are best for the package.
In short, Debian doesn't tell its developers what flags to use.